Semiconductor memory device and method of manufacturing the same

ABSTRACT

A semiconductor memory device has a silicon substrate  10.  A first embedded layer  11  is formed in the silicon substrate  10  under a p-well  18  in an area below a region where a drain  36  of a driver transistor  30  is located. The first embedded layer  11  makes a junction with the p-well  18.  Also, the first embedded layer  11  is formed below an n-well  16  and contacts the n-well  16.  When the drain  36  of the driver transistor  30  is at a voltage of 3V, α-ray may pass through the p-well  18,  the first embedded layer  11  and the silicon substrate  10.  As a result, electron-hole pairs are cut. Due to the presence of the p-n junction that is formed by the p-well  18  and the first embedded layer  11,  only electrons in the p-well  18  are drawn to the drain  36.  As a result, a fall in the drain voltage of 3V is reduced. As a consequence, the device structure makes it difficult to destroy retained data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory device and, moreparticularly, to an SRAM (Static Random Access Memory) and a method formanufacturing the same.

2. Description of the Related Art

FIG. 27 shows an equivalent circuit diagram of an SRAM memory cell. Aload transistor Q₅ and a driver transistor Q₃ form an inverter. A loadtransistor Q₆ and a driver transistor Q₄ form an inverter. The invertersare electrically connected to each other to form a flip-flop.

A transfer transistor Q₂ connects an output cell node 1000 of theinverter that is formed by the load transistor Q₆ and the drivertransistor Q₄ and a bit line (BL). A gate electrode of the transfertransistor Q₂ is electrically connected to a word line.

Source regions of the load transistors Q₅ and Q₆ electrically connect toa power supply line V_(DD). Source regions of the driver transistors Q₃and Q₄ electrically connect to a ground line V_(SS).

A transfer transistor Q₁ connects an output cell node 1002 of theinverter that is formed by the load transistor Q₅ and the drivertransistor Q₃ and a bit line (/BL). A gate electrode of the transfertransistor Q₁ is electrically connected to a word line.

The flip-flop retains a state in which the cell node 1000 is at avoltage of 3V, for example, and the cell node 1002 is at a voltage of0V, for example, as “1”, for example. Also, the flip-flop retains astate in which the cell node 1000 is at a voltage of 0V, for example,and the cell node 1002 is at a voltage of 3V, for example, as “0”, forexample.

An SRAM may suffer a problem of an α-ray soft error. Materials forwiring layers, molding resin, and the like contain a very small amountof radioactive substances. The radioactive substances generate α-rays.The α-ray soft error is a phenomenon in which retained data is destroyeddue to the α-ray. The destruction of retained data by the α-ray softerror will be described below in detail, with reference to theaccompanying figure.

FIG. 25 is a cross-sectional view of a silicon substrate 200 in whichthe load transistor Q₆ and the driver transistor Q₄ are formed. Thesilicon substrate 200 is of a p-type. An n-well 202 and a p-well 204 areformed adjacent to each other. A source 212 and a drain 214 of thedriver transistor Q₄ are formed in the p-well 204. A p-type well contactregion 216 is formed in the p-well 204. The well contact region 216 isisolated from the source 212 by a field oxide film 206. The well contactregion 216 and the source 212 are electrically connected to the groundline V_(SS).

A source 218 and a drain 220 of the load transistor Q₆ are formed in then-well 202. An n-type well contact region 222 is formed in the n-well202. The well contact region 222 is isolated from the source 218 by afield oxide film 210. The well contact region 222 and the source 218 areelectrically connected to the power supply line V_(DD). The drain 220 isisolated from the drain 214 by a filed oxide film 208.

Next, the destruction of retained data by the α-ray soft error will bedescribed with reference to FIGS. 25 and 26. As shown in FIG. 25, whenthe cell node 1000 is at 3V, for example, the drain 214 is at 3V, andthe p-well 204 is biased to the ground line V_(SS). Therefore, because adiode formed by the drain 214 and the p-well 204 is inversely biased, adepletion layer is formed.

In this state, if an α-ray passes through the drain 214 and the p-well204 and reaches the silicon substrate 200, the depletion layer of thediode is warped by the α-ray. As a result, electron-hole pairs are cutalong the pass of the α-ray. As shown in FIG. 26, the holes flow intothe well contact region 216 and into the ground line V_(SS). Theelectrons flow into the drain 214 that is at a high voltage. The flowsof the holes and the electrons lower the drain voltage. As a result, theretained data is destroyed. In other words, in this example, the stateof the cell node 1000 changes from 3V to 0V, and therefore the state “1”changes to “0”.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductormemory device with a structure that is difficult to cause harmfuleffects on the memory function and a method for manufacturing the same.

In accordance with one embodiment of the present invention, asemiconductor memory device has a semiconductor substrate defining amain surface, and a peripheral circuit region and an SRAM memory cellregion in the main surface. The semiconductor memory device comprises afirst well, a second well of a first conductivity type, a third well ofa second conductivity type, a device element isolation structure, anembedded layer of the second conductivity type, a driver transistor, aload transistor, and an impurity region of the second conductivity type.The first well is formed in the peripheral circuit region, and thesecond well is formed in the memory cell region. The second well isshallower than the first well. The driver transistor is formed in thesecond well. The impurity region is formed in the second well. Theimpurity region is a drain of the driver transistor. The impurity regionis an element that composes a cell node. The third well is formed in thememory cell region. The third well is shallower than the first well. Theload transistor is formed in the third well. The device elementisolation structure is formed in the memory cell region. The deviceelement isolation structure isolates the driver transistor from the loadtransistor. The second well and the third well are formed to extend to alocation under the device element isolation structure. The embeddedlayer is formed under the second well and under an area where at leastthe impurity region is located. The embedded layer forms a junction withthe second well. The embedded layer is fixed at a potential thatprevents carriers of the second conductivity type in the embedded layerfrom flowing into the second well.

The semiconductor memory device in accordance with the embodimentachieves the following advantages.

The miniaturization of a memory cell must address two conflictingissues. The length of the device element isolation structure (such as asemi-recessed LOCOS oxidation layer) of the memory cell area needs to beshortened in order to miniaturize the memory cell. On the other hand, toprevent the generation of a parasitic MOS leak current that causeslatch-up, the spacing between one well and source/drain of another wellformed adjacent to the one well needs to be longer than a certaindistance. Accordingly, when the length of the device element isolationstructure is shortened to miniaturize the memory cell, the spacingbetween one well and the source/drain of another well formed adjacent tothe one well may become too short to an extent that a parasitic MOS leakcurrent may be readily generated. However, in accordance with theembodiment of the present invention, the length of the device elementisolation structure can be reduced for the miniaturization of the memorycell, while the distance between one well and the source/drain ofanother well formed next to the one well can be prevented from becomingtoo short.

In a semiconductor memory device in accordance with one embodiment ofthe present invention, the wells in the peripheral circuit region andthe wells in the memory cell region may be different in depth. In otherwords, the second and third wells formed in the memory cell region areshallower than the first well formed in the peripheral circuit region.Accordingly, this structure can reduce an overlapped area between thesecond well and the third well beneath the device element isolationstructure. The reason for this will be described below in conjunctionwith the discussion of the embodiments. Accordingly, in a semiconductormemory device in accordance with the present invention, the length ofthe device element isolation structure can be reduced, while thedistance between one well and the source/drain of another well formednext to the one well can be prevented from becoming too short.

In accordance with another embodiment of the present invention, thesemiconductor memory device of the present invention further includes anembedded layer. The embedded layer is formed in an area under the secondwell where at least the impurity region is located. The embedded layerforms a junction with the second well. The embedded layer is fixed at apotential that prevents carriers of the second conductivity type in theembedded layer from flowing into the second well.

In the semiconductor memory device in accordance with the presentinvention, the embedded layer can prevent the generation of an α-raysoft error that may occur in the impurity region of the drivertransistor (an element that forms a cell node). For example, when thesemiconductor substrate is of a p-type, the embedded layer is an n-type,the second well is of a p-type, and the impurity region is of an n-type,the following phenomenon occurs.

Let us consider a situation when the impurity region of the drivertransistor (an element that forms a cell node) is, for example, at avoltage of 3V, and an α-ray passes through the impurity region of thedriver transistor, the second well, the embedded layer, and thesemiconductor substrate. As a result, the electron-hole pairs are cut.It is believed that only electrons in the second well are drawn to theimpurity region (element that forms a cell node) because of the presenceof the p-n junction between the second well and the embedded layer.Electrons in the embedded layer and in the semiconductor substrate donot flow into the second well, because the embedded layer is fixed at apositive potential (a potential that prevents the electrons from flowinginto the second well).

In the manner described above, only the electrons in the second well aredrawn to the impurity region (element that forms a cell node).Accordingly, the voltage at the impurity region (element that forms acell node) is difficult to become unstable, and, therefore, this makesthe retained data difficult to be destroyed.

When a first embedded layer is an n-type, the impurity concentration ofthe first embedded layer is preferably 5E12-5E13 cm⁻². This impurityconcentration is the same as the impurity concentration of a lowresistance layer of the n-type first well.

In the embodiments of the present invention, the cell node refers to anode at which a transfer transistor and an output of an inverter that isformed by a load transistor and a driver transistor are connected toeach other.

In accordance with the present invention, examples of the device elementisolation structure include a LOCOS oxide film, a semi-recessed LOCOSoxide film, or a shallow trench (the depth ranging between about 0.4 and0.8 μm).

In accordance with the present invention, the “source/drain” refers toat least one of a source and a drain.

In a semiconductor memory device in accordance with another embodimentof the present invention, the following structure may be added. Forexample, the semiconductor memory device of the present inventionincludes another impurity region of the first conductivity type. Theanother impurity region is formed in the third well. The anotherimpurity region is a drain of the load transistor. The another impurityregion is an element that forms the cell node. The embedded layer is notformed in an area under the third well where the another impurity regionis located. The semiconductor substrate is the first conductivity type.

As a result of the addition of the above-described structure, thegeneration of an α-ray soft error that may occur in the another impurityregion of the load transistor (an element that forms a cell node) can beprevented. For example, when the semiconductor substrate is of a p-type,the embedded layer is n-type, the third well is of an n-type, and theanother impurity region is of a p-type, the following phenomenon occurs.

Let us consider a situation where the another impurity region of theload transistor (an element that forms a cell node) is, for example, ata voltage of 0V, and an α-ray passes through the another impurity regionof the load transistor, the third well, and the semiconductor substrate,with the result that the electron-hole pairs are cut. As a result, it isbelieved that only holes in the third well are drawn to the anotherimpurity region (element that forms a cell node) because of the presenceof the p-n junction between the third well and the semiconductorsubstrate.

In this manner, since the embedded layer is not formed in an area underthe third well where the another impurity region is located, only theholes generated in the third well among the holes generated by the α-rayresult in a soft error. Because the third well is relatively shallow,relatively few holes are generated therein. As a result, the voltage ofthe another impurity region is difficult to become unstable, and thismakes the retained data difficult to be destroyed.

In the semiconductor memory device in accordance with the presentinvention, the following structure may be added. The semiconductormemory device of the present invention further includes a well contactregion of the second conductivity type. The well contact region isformed in the third well, the well contact region is a contact regionthat fixes a well potential of the third well, and the embedded layercontacts the third well.

As a result of the addition of the above-described structure, thepotential of the embedded layer can be fixed at a potential that canprevent the carriers of the second conductivity type in the embeddedlayer from flowing into the second well. For example, when a positivevoltage is applied to the well contact region, electrons generated inthe embedded layer in the example described above are drawn from theembedded layer through the third well to the well contact region.

In the semiconductor memory device in accordance with the presentinvention, the following structure may be added. For example, thesemiconductor memory device in accordance with the present inventionincludes another embedded layer of the first conductivity type. Theanother embedded layer is formed under the second well, and the anotherembedded layer contacts the second well.

As a result of the addition of the structure described above, thegeneration of latch-up can be prevented. More specifically, theconductivity type of the another embedded layer is the same as theconductivity type of the second well, and, therefore, the resistance ofthe second well is lowered. This contributes to the prevention of thegeneration of latch-up. In one embodiment, when the another embeddedlayer is a p-type, the another embedded layer may have an impurityconcentration of 5E12-5E13 cm⁻².

In the semiconductor memory device in accordance with the presentinvention, the first well, the second well, and the third well may beretrograded wells. The retrograded well is a well that is formed by ahigh-energy ion implantation without using a thermal diffusion.

Each of the first well, the second well, and the third well, that areretrograded wells, has a first concentration layer, a secondconcentration layer, and a third concentration layer in the order from atop layer. The first well further has a fourth concentration layer underthe third concentration layer. The first concentration layer may be, forexample, a channel dope layer that adjusts the threshold voltage (Vth)of the transistor. The second concentration layer may be, for example, apunch-through stopper layer that suppresses the short channel effect ofthe transistor. The third concentration layer may be, for example, achannel-cut layer that prevents the operation of a parasitic transistor.The fourth concentration layer may be, for example, a low resistancelayer that lowers the well resistance. In one embodiment, in the secondand third wells, the first concentration layer may be a channel dopelayer, the second concentration layer may be a channel stopper layer,and the third concentration layer may be a channel cut layer, forexample.

In the semiconductor memory device in accordance with the presentinvention, the length of the device element isolation region thatisolates the wells in the memory cell region may fall within a range ofabout 0.2 μm-1.6 μm. The border between the second well and the thirdwell needs to be located below the device element isolation structure.When the resist is patterned, a positional alignment error may occur.Accordingly, the device element isolation structure requires a certainminimum length. The minimum length of the structure may be about 0.2 μm.Also, when the device element isolation structure has a length longerthan about 1.6 μm, the size of the memory cell becomes too large.

In the semiconductor memory device in accordance with the presentinvention, the second well and the third well may have a depth of about0.5-1.2 μm. When the second and third wells are shallower than about 0.5μm, the device element isolation structure becomes deeper than thewells. This presents a problem in designing as to how a well contactregion for fixing the potential of the well is formed. When the secondand third wells are deeper than about 1.2 μm, the overlapped areabetween the second well and the third well, beneath the device elementisolation structure, becomes greater.

In the semiconductor memory device in accordance with the presentinvention, the semiconductor substrate may be of a p-type. As a result,the p-type wells in the memory cell region are connected to one anotherthrough the semiconductor substrate, and, therefore, the resistance ofthe wells is lowered. As a result, this suppresses an increase in thesubstrate potential of a region, where the n-channel transistor (havinga relatively large substrate current compared to the p-channeltransistor) is formed.

In accordance with another embodiment of the present invention, a methodis provided for manufacturing a semiconductor memory device having asemiconductor substrate defining a main surface, and a peripheralcircuit region and an SRAM memory cell region in the main surface. Themethod includes the following steps. (a) A device element isolationstructure is formed in the main surface. (b) A first well is formed byion-implanting an impurity in the peripheral circuit region. (c) Anembedded layer of a second conductivity type is formed by ion-implantingan impurity in the memory cell region. (d) A third well of the secondconductivity is formed by ion-implanting an impurity in the memory cellregion, wherein the third well is shallower than the first well, formedto extend to a location under the device element isolation structure,and in contact with the embedded layer. (e) A second well of the firstconductivity type is formed by ion-implanting an impurity in the memorycell region. The second well is in contact with the third well in anarea lower than the device element isolation structure, and is formedover the embedded layer. Also, the second well forms a junction with theembedded layer. (f) A driver transistor is formed in the second well.Among the impurity regions of the driver transistor, an impurity regionthat forms an element composing a cell node is formed in a manner thatthe embedded layer is located under the impurity region. (g) A loadtransistor is formed in the third well.

In the above-described embodiment of the present invention, a method isprovided for manufacturing a semiconductor memory device that canprevent the distance between one well and source/drain of another welladjacent to the one well from becoming too short without excessivelyelongating the device element isolation structure.

In accordance with the present invention, an embedded layer is furtherformed, as described below. The embedded layer is formed in an areaunder the second well, under a region where the impurity region of thedriver transistor (element that forms a cell node) is located, and formsa junction with the second well. Also, the embedded layer contacts thethird well.

In the method for manufacturing the semiconductor memory device inaccordance with the present invention, in step (b) and step (c), aresist pattern having a thickness of about 3.0 μm-8.0 μm is used as amask, and in step (d) and step (e), a resist pattern having a thicknessof about 1.2 μm-2.5 μm is used as a mask.

If the thickness of the resist pattern is thinner than about 3.0 μm insteps (b) and (c), the impurity penetrates the resist pattern when theion implantation is performed to form a retrograded well. If thethickness of the resist pattern is thicker than about 8.0 μm, control ofthe configuration of the end portion of the resist pattern is difficult.As a result, a problem arises in which the length of the device elementisolation structure needs to be increased.

If the thickness of the resist pattern is smaller than about 1.2 μm insteps (d) and (e), the impurity penetrates the resist pattern when theion implantation is conducted to form a retrograded well. If thethickness of the resist pattern is greater than about 2.5 μm, thecontrol of the configuration of the end portion of the resist pattern isdifficult. As a result, a problem arises in which the overlapped areabetween the second well and the third well below the device elementisolation structure becomes greater.

Either a positive resist or a negative resist works as the resistpattern in steps (b) and (c). Also, a positive resist or a negativeresist works as the resist pattern in steps (d) and (e). However, morepreferably, a positive resist may be used. This is because the positiveresist outperforms the negative resist in the control of the verticalconfiguration of the end portion of the resist pattern and thedimensional control of the resist pattern.

In the method for manufacturing the semiconductor memory device inaccordance with the present invention, the step of implanting ions instep (b) includes the step of implanting ions in step (c).

In the method for manufacturing the semiconductor memory device inaccordance with another embodiment of the present invention, thefollowing features may be added where the first well is a twin-wellhaving a well of a first conductivity type and a well of a secondconductivity type. Step (b) includes implanting ions in a region wherethe well of the first conductivity type is formed and implanting ions ina region where the well of the second conductivity type is formed, andstep (d) includes implanting ions three times in a region where thethird well is formed and in a region where the well of the secondconductivity type in the peripheral circuit region is formed. The stepof implanting ions three times forms the third well having a thirdconcentration layer, a second concentration layer and a firstconcentration layer, in the order from a bottom layer, in the memorycell region. Step (b) and the step of implanting ions three times formthe well of the second conductivity type having a fourth concentrationlayer, a third concentration layer, a second concentration layer and afirst concentration layer, in the order from a bottom layer, in theperipheral circuit region. Furthermore, step (e) includes implantingions three times in a region where the second well is formed and aregion where the well of the first conductivity type in the peripheralcircuit region is formed. The step of implanting ions three times formsthe second well having a third concentration layer, a secondconcentration layer and a first concentration layer, in the order from abottom layer, in the memory cell region; and step (b) and the step ofimplanting ions three times form the well of the first conductivity typehaving a fourth concentration layer, a third concentration layer, asecond concentration layer and a first concentration layer, in the orderfrom a bottom layer, in the peripheral circuit region.

In the method for manufacturing the semiconductor memory device inaccordance with the present invention, the following step (h) may beadded. For example, step (h) includes forming another embedded layer ofthe first conductivity type under a region where the second well isformed.

According to this manufacturing method, the semiconductor memory devicehas another embedded layer of the first conductivity type, wherein theanother embedded layer is formed under the second well, and the anotherembedded layer contacts the second well.

In the method for manufacturing the semiconductor memory device inaccordance with the present invention, the step of implanting ions instep (b) includes implanting ions in step (h).

Other features and advantages of the invention will be apparent from thefollowing detailed description, taken in conjunction with theaccompanying drawings which illustrate, by way of example, variousfeatures of embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor memory device inaccordance with a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a semiconductor memory device, whichis used to explain the effects of the semiconductor memory device inaccordance with a first embodiment of the present invention.

FIG. 3 is a cross-sectional view of a semiconductor memory device, whichis used to explain the effects of the semiconductor memory device inaccordance with a first embodiment of the present invention.

FIG. 4 is a cross-sectional view, which is used to describe a first stepof a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 5 is a cross-sectional view, which is used to describe a secondstep of a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 6 is a cross-sectional view, which is used to describe a third stepof a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 7 is a cross-sectional view, which is used to describe a fourthstep of a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 8 is a cross-sectional view, which is used to describe a fifth stepof a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 9 is a cross-sectional view, which is used to describe a sixth stepof a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 10 is a cross-sectional view, which is used to describe a seventhstep of a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 11 is a cross-sectional view, which is used to describe a eighthstep of a method for manufacturing the semiconductor memory device inaccordance with the first embodiment of the present invention.

FIG. 12 is a cross-sectional view, which is used to describe changes inthe shape of a resist pattern.

FIG. 13 is a cross-sectional view, which is used to describe a latchupphenomenon.

FIG. 14 is a cross-sectional view of a semiconductor memory device inaccordance with a second embodiment of the present invention.

FIG. 15 is a cross-sectional view of a semiconductor memory device,which is used to explain the effects of the semiconductor memory devicein accordance with the second embodiment of the present invention.

FIG. 16 is a cross-sectional view of a semiconductor memory device,which is used to explain the effects of the semiconductor memory devicein accordance with the second embodiment of the present invention.

FIG. 17 is a cross-sectional view, which is used to describe a firststep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 18 is a cross-sectional view, which is used to describe a secondstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 19 is a cross-sectional view, which is used to describe a thirdstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 20 is a cross-sectional view, which is used to describe a fourthstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 21 is a cross-sectional view, which is used to describe a fifthstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 22 is a cross-sectional view, which is used to describe a sixthstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 23 is a cross-sectional view, which is used to describe a seventhstep of a method for manufacturing the semiconductor memory device inaccordance with the second embodiment of the present invention.

FIG. 24 is a cross-sectional view of a semiconductor memory device inaccordance with a third embodiment of the present invention.

FIG. 25 is a first cross-sectional view, which is used to explain theα-ray soft error in an SRAM.

FIG. 26 is a second cross-sectional view, which is used to explain theα-ray soft error in an SRAM.

FIG. 27 shows an equivalent circuit of an SRAM.

DESCRIPTION OF PREFERRED EMBODIMENTS

[First Embodiment]

[Description of the Structure of Device]

FIG. 1 is a cross-sectional view of a semiconductor memory device inaccordance with a first embodiment of the present invention. Thesemiconductor memory device 1 is an SRAM. Referring to FIG. 1, the mainsurface of a p-type silicon substrate 10, as one example ofsemiconductor substrate, is divided into a memory cell region 12 and aperipheral circuit region 14.

An n-well 16 and a p-well 18 are formed in the memory cell area 12. Then-well 16 and the p-well 18 are retrograded wells. The n-well 16 and thep-well 18 overlap each other in a border between the n-well 16 and thep-well 18. A semi-recessed LOCOS oxidation layer 20 is formed on theborder. The depth D of the n-well 16 and the depth D of the p-well 18are about 0.5 μm-1.2 μm. The length L of the semi-recessed LOCOSoxidation layer 20 is about 0.2 μm-1.6 μm.

An n-type first embedded layer 11 and a p-type second embedded layer 13are embedded in the silicon substrate 10 in the memory cell region 12.The location of the first embedded layer 11 will be described in detail.The first embedded layer 11 is formed below the p-type well 18 and in anarea below a region where the driver transistor 30 is located and joinedwith the p-well 18. First embedded layer 11 is also formed under then-well 16 and in contact with the n-well 16.

Next, the location of the second embedded layer 13 will be described indetail below. The second embedded layer 13 is formed in an area below aregion where the transfer transistor 31 is located, and is in contactwith the p-well 18.

A load transistor 22 is formed in the n-well 16. The load transistor 22is a p-channel transistor. The load transistor 22 has a gate electrode24, a p-type source 26 and a p-type drain 28. The source 26 connects tothe power supply line V_(DD). An n-type well contact region 27 is formedin the n-well 16. The well contact region 27 connects to a wiring thatfixes the potential of the n-well 16. In this embodiment, the wellcontact region 27 connects to the power supply line V_(DD). The wellcontact region 27 is isolated from the drain 28 by a semi-recessed LOCOSoxide film 29.

A driver transistor 30 is formed in the p-well 18. The driver transistor30 is an n-channel transistor. The driver transistor 30 has a gateelectrode 32, a n-type source 34 and an n-type drain 36. The source 34connects to the ground line V_(SS). The drain 36 connects to the drain28 of the load transistor 22. A transfer transistor 31 is formed in thep-well 18. The transfer transistor 31 is an n-channel transistor. Thetransfer transistor 31 has a gate electrode 35, a source 33 and a drain37. The source 33 and the drain 36 are formed from the same impurityregion. The drain 37 connects to a bit line BL.

Also, a p-type well contact region 38 is formed in the p-well 18. Thewell contact region 38 connects to a wiring line that fixes thepotential of the p-well 18. In the present embodiment, the well contactregion 38 connects to the ground line V_(SS). The well contact region 38is isolated from the drain 37 by a semi-recessed LOCOS oxide film 40.

An inverter that is formed from the load transistor 22 and the drivertransistor 30 is electrically connected to the transfer transistor 31 bya cell node 39. The drain 28, the drain 36 and the source 33 are partsof the cell note 39.

An n-well 42 and a p-well 44 adjacent thereto are formed in theperipheral circuit region 14. The n-well 42 and the p-well 44 areretrograded wells. The n-well 42 and the p-well 44 are overlapped eachother in a border region between these wells. A semi-recessed LOCOSoxide film 46 is formed over the border.

A p-channel transistor 48 is formed in the n-well 42. The p-channeltransistor 48 has a gate 50, and p-type source/drain 52 and 54. Ann-channel transistor 56 is formed in the p-well 44. The n-channeltransistor 56 has a gate electrode and two n-type source/drains. FIG. 1shows a part of the gate electrode 58 and one source/drain 60. Asemi-recessed LOCOS oxide film 64 is formed on the surface in a borderregion between the memory cell region 12 and the peripheral circuitregion 14.

{Description of Method for Manufacturing Devices}

A method for manufacturing the semiconductor memory device 1 inaccordance with one embodiment of the present invention will bedescribed below with reference to FIGS. 4-11. A plurality of trenches(not shown) are formed in the main surface of the silicon substrate 10that has an anti-oxidation layer, such as silicon nitride, formedthereon. As shown in FIG. 4, oxide films are formed in the trenchesusing the LOCOS technique. Semi-recessed LOCOS oxidation layers 29, 20,40, 64 and 46, each having a thickness in a range of about 0.2-0.7 μm,are thus formed.

Referring to FIG. 5, a resist pattern 92 having a thickness in a rangeof about 3.0-8.0 μm is formed such that an n-well forming region in theperipheral circuit region 14 and a first embedded layer forming regionin the memory cell region 12 are exposed. Phosphorous is implanted inthe silicon substrate 10, using the resist pattern 92 as a mask, to forma low resistance layer 94 and a first embedded layer 11. Theimplantation energy ranges from 500 KeV to 3 MeV with a dose rangingfrom 5E12 to 5E13 cm⁻².

Referring to FIG. 6, a resist pattern 96 having a thickness in a rangeof 3.0 μm-8.0 μm is formed such that a p-well forming region and asecond embedded layer forming region in the peripheral circuit region 14are exposed. Boron is implanted in the silicon substrate 10, using theresist pattern 96 as a mask, to form a low resistance layer 98 and asecond embedded layer 13. The implantation energy ranges from 300 KeV to2 MeV with a dose ranging from 5E12 to 5E13 cm⁻².

Referring to FIG. 7, a resist pattern 100 having a thickness in a rangeof about 1.2 μm-2.5 μm is formed such that n-well forming regions in thememory cell region 12 and the peripheral circuit region 14 are exposed.Phosphorous is implanted in the silicon substrate 10, using the resistpattern 100 as a mask, to initially form channel cut layers 102 and 104.The implantation energy ranges from 200 KeV to 500 KeV with a doseranging from 3E12 to 2E13 cm⁻².

Next, phosphorous is implanted in the silicon substrate 10, using theresist pattern 100 as a mask, to form punch-through stopper layers 106and 108. The implantation energy ranges from 100 KeV to 200 KeV with adose ranging from 2E12 to 2E13 cm⁻². The punch-through stopper layers106 and 108 may be formed using arsenic. In this case, the implantationenergy ranges from 150 KeV to 300 KeV with a dose ranging from 2E12 to1E13 cm⁻².

Then, ions are implanted in the silicon substrate 10, using the resistpattern 100 as a mask, to form channel dope layers 110 and 112. Each ofthe channel dope layers 110 and 112 may be formed by any one of thefollowing impurities (a), (b) and (c), alone or in combination:

(a) phosphorous (impurity ions): 20 KeV to 100 KeV (implantation energy)with 1E12 to 1E13 cm⁻² (dose);

(b) boron difluoride (impurity ions): 30 KeV to 100 KeV (implantationenergy) with 1E12 to 1E13 cm⁻² (dose); and

(c) boron (impurity ions): 10 KeV to 50 KeV (implantation energy) with1E12 to 1E13 cm⁻² (dose).

The above-described steps complete the formation of the n-well 16including the channel cut layer 102, the punch-through stopper layer 106and the channel dope layer 110 in the memory cell region 12. In theperipheral circuit region 14, the above-described steps complete theformation of the n-well 42 including the low resistance layer 94, thechannel cut layer 104, the punch-through stopper layer 108 and thechannel dope layer 112.

As shown in FIG. 8, a resist pattern 114 having a thickness in a rangeof 1.2 μm-2.5 μm is formed such that p-well forming regions in thememory cell region 12 and the peripheral circuit region 14 are exposed.Boron is implanted in the silicon substrate 10, using the resist pattern114 as a mask, to form channel cut layers 116 and 118. The implantationenergy ranges from 100 KeV to 300 KeV with a dose ranging from 3E12 to2E13 cm⁻².

Boron is implanted in the silicon substrate 10, using the resist pattern114 as a mask, to form punch-through stopper layers 120 and 122. Theimplantation energy ranges from 50 KeV to 200 KeV with a dose rangingfrom 2E12 to 1E13 cm⁻². The punch-through stopper layers 120 and 122overlap with the channel cut layers 116 and 118 depending on theimplanting energy. In one embodiment, one layer that functions as thetwo layers may be formed with one ion implantation.

Then, ions are implanted in the silicon substrate 10, using the resistpattern 114 as a mask, to form channel dope layers 124 and 126. Each ofthe channel dope layers 124 and 126 may be formed by any one of thefollowing impurities (a), (b) and (c), alone or in combination:

(a) phosphorous (impurity ions): 20 KeV to 100 KeV (implantation energy)with 1E12 to 1E13 cm⁻² (dose);

(b) boron difluoride (impurity ions): 30 KeV to 150 KeV (implantationenergy) with 1E12 to 1E13 cm⁻² (dose); and

(c) boron (impurity ions): 10 KeV to 50 KeV (implantation energy) with1E12 to 1E13 cm⁻² (dose).

The above-described steps complete the formation of the p-well 18including the channel cut layer 116, the punch-through stopper layer 120and the channel dope layer 124 in the memory cell region 12. In theperipheral circuit region 14, the above-described steps complete theformation of the p-well 44 including the low resistance layer 98, thechannel cut layer 118, the punch-through stopper layer 122 and thechannel dope layer 126.

FIG. 9 shows the silicon substrate 10 in a state in which the resistpattern 114 is removed from the silicon substrate 10 shown in FIG. 8. InFIG. 9, the illustration of lines for the channel cut layers 102, 104,116 and 118, the punch-through stopper layers 106, 108, 120 and 122, andthe channel dope layers 110, 112, 124 and 126 is omitted.

Referring to FIG. 10, gate electrodes 24, 32, 35, 50 and 58 are formed,using the ordinary gate electrode forming technique. A resist pattern 86is formed such that n-type source and drain forming regions in thememory cell region 12 and the peripheral circuit region 14, and n-typewell contact forming regions in the memory cell region 12, are exposed.For example, phosphorous is implanted in the silicon substrate 10, usingthe resist pattern 86 and the gate electrodes 32, 35 and 58 as masks, toform a source 34, a drain 36, a source 33, a drain 37, a source/drain60, and a well contact region 27.

As shown in FIG. 11, a resist pattern 84 is formed such that p-typesource and drain forming regions in the memory cell region 12 and theperipheral circuit region 14, and p-type well contact forming regions inthe memory cell region 12, are exposed. For example, boron is implantedin the silicon substrate 10, using the resist pattern 84 and the gateelectrodes 24 and 50 as masks, to form a drain 28, a source 26,source/drain 52 and 54, and a well contact region 38. The structureshown in FIG. 1 is completed, using the ordinary wiring connectiontechnique.

{Description of Effects}

(Effect 1)

In the semiconductor memory device 1 as shown in FIG. 1, the firstembedded layer 11 is formed under the p-well 18 in an area below aregion where the drain 36 of the driver transistor 30 is located, andmakes a junction with the p-well 18. Also, the first embedded layer 11is formed below the n-well 16 and contacts the n-well 16. As a result ofthis structure, the following effects are obtained.

Referring to FIG. 2, let us assume that, when the cell node is at 3V(namely, when the drain 36 of the driver transistor 30 is at a voltageof 3V), α-ray passes through the drain 36, the p-well 18, the firstembedded layer 11 and the silicon substrate 10, and as a result,electron-hole pairs are cut. Let us assume that the ground line V_(SS)is at 0V and the power supply line V_(DD) is at 3V.

The well contact region 27 connects to the power supply line V_(DD).Therefore, the first embedded layer 11 has a positive potential (apotential that prevents electrons from flowing in the p-well 18).Accordingly, as shown in FIG. 3, electrons in the first embedded layer11 and the silicon substrate 10 flow from the first embedded layer 11through the n-well 16, and are drawn to the well contact region 27.Therefore, only electrons in the p-well 18 are drawn to the drain 36.

As shown in FIG. 26, in the conventional semiconductor memory device,all of the electrons along a track in which α-ray passes flow in thedrain 214. The distance of the track is equal to the depth of the p-well204 plus the depth of the silicon substrate 200, which is 20-100 μm. Incontrast, in the semiconductor memory device 1 shown in FIG. 3, thedistance of the track is equal to only the depth of the p-well 18, whichis about 0.5-1.2 μm. As a result, in the semiconductor memory device 1shown in FIG. 3, the fall of the drain voltage of 3V is smaller. As aresult, the retained data is more difficult to be destroyed. Holes aredrawn to the ground line V_(SS) that connects to the silicon substrate10 and the ground line V_(SS) that connects to the well contact region38. Therefore, they do not cause deteriorating effects on the dataretaining function.

It is noted that the first embedded layer 11 may be formed under thep-well 18 and only below a region where the n-type drain 36 is located.Such a structure also provides the same effects as described above. Thesame effects are obtained by embodiments that will be described below.

By contacting the first embedded layer 11 with the n-well 16, thepotential on the first embedded layer 11 is fixed at a value that canprevent electrons in the first embedded layer 11 from flowing into thep-well 18. The first embedded layer 11 may not be contacted with then-well 16, and may be contacted with another well, to set the firstembedded layer 11 at such a potential. The same principle applies toembodiments that will be described below.

(Effect 2)

In the semiconductor memory device 1 shown in FIG. 1, the p-type secondembedded layer 13 is provided below the p-well 18. As a result, theresistance of the p-well 18 can be lowered. This contributes to theprevention of the generation of latchup.

(Effect 3)

As shown in FIG. 1, the silicon substrate 10 is of p-type. Therefore,the p-wells in the memory cell region 12 connect to one another throughthe silicon substrate 10, so that the well resistance is lowered. As aresult, a rise of the substrate potential can be reduced in a regionthat forms the n-channel transistor that has a relatively largesubstrate current compared to the p-channel transistor in the memorycell region 12.

(Effect 4)

In the semiconductor memory device 1, as shown in FIG. 1, the depth ofthe n-well 16 and the p-well 18 in the memory cell region 12 is smallerthan the depth of the n-well 42 and the p-well 44 in the peripheralcircuit region 14. This structure lowers the possibility of generationof latch-up, and reduces the length of the device element isolationstructure (semi-recessed LOCOS oxide film 20) between the n-well 16 andthe p-well 18 in the memory cell region 12. The reasons will bedescribed below.

(A) The latch-up occurs when the product of a leak current of aparasitic MOS or a substrate current with the substrate resistancebecomes a certain value or greater. To reduce the substrate resistance,deep wells need to be formed. Since the thickness of a resist pattern isproportional to the depth of a well, the resist pattern needs to bethicker in order to form a deeper well.

However, if wells are formed by thick resist patterns, for example, then-well 42 and the p-well 44 overlap each other in a large area below thesemi-recessed LOCOS oxide film 46 in the peripheral circuit region 14,as shown in FIG. 1. The reason for this will be discussed with referenceto FIG. 12.

(B) FIG. 12 shows a state in which the resist pattern 132 is formed onthe device element isolation structure 130. FIG. 12 shows a resist endportion 136, namely, an end portion of the resist pattern 132. FIG. 12also shows a designed resist pattern 134 having a designed resist endportion 138.

The letter a represents a receding distance due to the proximity effectand loading effect. The letter b represents shrinkage at the upper edgeof the resist when the resist pattern is baked. The top edge of theresist end portion 136 recedes by an amount of a+b from the resist endportion 138.

Reference numeral 140 denotes a well end portion when the well is to beformed with the designed resist pattern 134 as a mask. Reference numeral142 denotes a well end portion when the well is actually formed with theresist pattern 132 as a mask. The upper edge of the well end portion 142advances by an amount c from the upper edge of the well end portion 140under the influence of the recession of the top edge of the resist endportion 136. This increases the intrusion of ions into the adjacentwell, thereby expanding the overlapped area. The effect of this expandedoverlapped area is discussed with reference to FIG. 13.

When a p-well 150 and an n-well 152 overlap each other in a large area,as shown in FIG. 8, one or both of the distance d between a p-typesource/drain 154 and the p-well 150 and the distance e between an n-typesource/drain 156 and the n-well 152 is shortened depending on the amountof ion implantation dose during the well formation. When this distanceshortens, for example, when the distance e shortens, a leakage currentof a parasitic MOS transistor tends to flow to the n-well 152 from then-type source/drain 156 through the p-well 150. This is attributable toa shortened effective channel length of the parasitic MOS transistor.The leakage current in the parasitic MOS transistor may work as atrigger current for latchup.

If a well is formed using a thin resist pattern, for example, having athickness of about 2 μm or less, the overlapped area becomes to berelatively small. This is because neither the receding distance a at theresist end portion nor the shrinkage b at the top end edge of the resistoccur in the end portion of the resist pattern.

When the length f of the device element isolation structure 158 is madegreater, the distances d and e may not become shorter. In the peripheralcircuit region having space margin available, the length f of the deviceelement isolation structure 158 may be made greater. However, if thelength f of the device element isolation structure 158 is made greaterin the memory cell region where no additional space is available, theminiaturization of the memory cell may not be achieved.

(C) The substrate current is relatively large in the peripheral circuitregion, because transistors having wide channel widths and largedrivability are arranged in the peripheral circuit region. To preventlatchup, the substrate resistance needs to be reduced. For this reason,the well depth has to be increased in the peripheral circuit region. Onthe other hand, the substrate current is relatively small in the memorycell region, because the size of a transistor constituting a memory cellis small. As a result, the latchup is prevented without greatly reducingthe substrate resistance. Therefore, the depth of a well can be madesmall in the memory cell.

For the reasons discussed above, the well depth is set to be shallow inthe memory cell region. This reduces an overlap between the p-well andthe n-well, and reduces the possibility of the generation of latchup.

[Second Embodiment]

{Description of Device Structure}

FIG. 14 is a cross-sectional view of a semiconductor memory device inaccordance with a second embodiment of the present invention. In asemiconductor memory device 3 shown in FIG. 14, the same referencenumerals are used for elements having the same functions as those of theelements of the semiconductor memory device 1 in accordance with thefirst embodiment of the present invention shown in FIG. 1. Thesemiconductor memory device 3 is different from the semiconductor memorydevice 1 in some respects. The differences will be described below, butthe description of the elements having the same functions will beomitted.

A first embedded layer 11 is formed below a p-well 18 and an n-well 16,and extends from an area where a gate electrode 35 is located to an areawhere a semi-recessed LOCOS oxide film 20 is located. The first embeddedlayer 11 is not formed in an area under the n-well 16 where the p-typedrain 28 is located. Also, the first embedded layer 11 has an n-typeimpurity of a low concentration.

Since the first embedded layer 11 is formed in an area extending to alocation below the gate electrode 35, a second embedded layer 13 isformed in an area shifted accordingly. Also, the second embedded layer13 has a p-type impurity of a low concentration.

{Description of Method for Manufacturing Device}

A method for manufacturing the semiconductor memory device 3 inaccordance with a second embodiment of the present invention will bedescribed with reference to FIGS. 17-23.

In a similar manner conducted for manufacturing the semiconductor memorydevice 1 of the first embodiment, the step described with reference toFIG. 4 is conducted.

Then, a step shown FIG. 17 is conducted. The step shown in FIG. 17corresponds to the step shown in FIG. 5. However, the opening 92 a ofthe resist pattern 92 is used for forming the first embedded layer 11 ofthe semiconductor memory device 3. Ions are implanted in the siliconsubstrate 10, using the resist pattern 92 as a mask, to form the lowresistance layer 94 and the first embedded layer 11. The same formingconditions described with reference to FIG. 5 are used.

Then, a step shown in FIG. 18 is conducted. The step shown in FIG. 18corresponds to the step shown in FIG. 6. However, the opening 96 a ofthe resist pattern 96 is used for forming the third embedded layer 13 ofthe semiconductor memory device 3. Ions are implanted in the siliconsubstrate 10, using the resist pattern 96 as a mask, to form the lowresistance layer 98 and the second embedded layer 13. The same formingconditions described with reference to FIG. 6 are used.

Next, a step shown in FIG. 19 is conducted. The step shown in FIG. 19 isthe same as the step shown in FIG. 7. Accordingly, the descriptionthereof is omitted.

Next, a step shown in FIG. 20 is conducted. The step shown in FIG. 20 isthe same as the step shown in FIG. 8. Accordingly, the descriptionthereof is omitted.

Next, a step shown in FIG. 21 is conducted. The step shown in FIG. 21 isthe same as the step shown in FIG. 9. Accordingly, the descriptionthereof is omitted.

Next, a step shown in FIG. 22 is conducted. The step shown in FIG. 22 isthe same as the step shown in FIG. 10. Accordingly, the descriptionthereof is omitted.

Next, a step shown in FIG. 23 is conducted. The step shown in FIG. 23 isthe same as the step shown in FIG. 11. Accordingly, the descriptionthereof is omitted. The ordinary wiring connection technique is used tocomplete the semiconductor memory device 3 shown in FIG. 14.

{Description of Effects}

The semiconductor memory device 3 of the second embodiment achievessubstantially the same effects (i.e., Effect 1-Effect 4) achieved by thesemiconductor memory device of the first embodiment described above. Inaddition, the following effects are also attained.

The semiconductor memory device 3 shown in FIG. 14 can prevent the α-raysoft error that may be generated in the drain 28 of the load transistor22 (that is an element composing a cell node 39). This phenomenon willbe described with reference to FIG. 15 and FIG. 16.

As shown in FIG. 15, let us assume that the cell node 39 is at 0V (inother words, the drain 28 of the load transistor 22 is at 0V). Since thedrain 28 is at 0V, and the n-well 16 is biased to the power supply lineV_(DD) through the well contact region 27, a depletion layer is formedin a junction area between the drain 28 and the n-well 16. Let us assumethat α-ray is injected in this state. The α-ray passes through then-well 16 and the silicon substrate 10, and electron-hole pairs would becut. The silicon substrate 10 is biased to the ground line V_(SS).

As shown in FIG. 16, holes in the silicon substrate 10 are drawn to theground line V_(SS). The holes that are drawn to the drain 28 are thosein the n-well 16. The depth of the n-well 16 in the semiconductor memorydevice 3 is about 0.5-1.2 μm. As a result, a rise in the potential ofthe drain region 28 can be reduced. For the reasons described above, theretained data is difficult to be destroyed even when the cell node 39 isat 0V.

[Third Embodiment]

FIG. 24 is a cross-sectional view of a semiconductor memory device inaccordance with a third embodiment of the present invention. In asemiconductor memory device 5 shown in FIG. 24, the same referencenumerals are used for elements having the same functions as those of theelements of the semiconductor memory device 3 in accordance with thesecond embodiment of the present invention shown in FIG. 14. Thesemiconductor memory device 5 is different from the semiconductor memorydevice 3 in some respects. The differences will be described below, butthe description of the elements having the same functions will beomitted.

The semiconductor memory device 5 does not have a second embedded layer13. Therefore, the semiconductor memory device 5 has the followingcharacteristic effects.

In the semiconductor memory device 5, the memory cell region 12 has onlyone embedded layer, that is the first embedded layer 11. Accordingly,this structure provides a higher freedom in designing a pattern (area,location, etc.) for the first embedded layer 11. For example, the areafor the first embedded layer 11 shown in FIG. 24 can be extended to anarea below the drain 37 of the transfer transistor 35. By thisstructure, the workability of the resist pattern that is used for thefirst embedded layer 11 shown in FIG. 17 is improved. Accordingly, theapplicability of the process to a smaller memory cell can be improved.

The semiconductor memory device 5 of the third embodiment achievessubstantially the same effects (i.e., Effect 1, Effect 3 and Effect 4)of the semiconductor memory device 1 of the first embodiment describedabove.

The semiconductor memory device 5 of the third embodiment can preventthe α-ray soft error that may be generated in the drain 28 of the loadtransistor 22 (that is an element composing a cell node 39), in asimilar manner described above with reference to the semiconductormemory device 3 of the second embodiment.

The semiconductor memory device 5 may be manufactured by the samemanufacturing method employed for manufacturing the semiconductor memorydevice 3, as shown in FIG. 17 through FIG. 23. While the descriptionabove refers to particular embodiments of the present invention, it willbe understood that many modifications may be made without departing fromthe spirit thereof. The accompanying claims are intended to cover suchmodifications as would fall within the true scope and spirit of thepresent invention.

The presently disclosed embodiments are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A semiconductor memory device having asemiconductor substrate defining a main surface, and a peripheralcircuit region and an SRAM memory cell region in the main surface,comprising: a first well, a second well of a first conductivity type, athird well of a second conductivity type, a device element isolationstructure, an embedded layer of the second conductivity type, a drivertransistor, a load transistor and an impurity region of the secondconductivity type, wherein the first well is formed in the peripheralcircuit region, the second well is formed in the memory cell region, thesecond well is shallower than the first well, the driver transistor isformed in the second well, the impurity region is formed in the secondwell, the impurity region is a drain of the driver transistor, theimpurity region is an element that composes a cell node, the third wellis formed in the memory cell region, the third well is shallower thanthe first well the load transistor is formed in the third well, thedevice element isolation structure is formed in the memory cell region,the device element isolation structure isolates the driver transistorfrom the load transistor, the second well and the third well are formedto extend to a location under the device element isolation structure,the embedded layer is formed under the second well and below an areawhere at least the impurity region is located, the embedded layer formsa junction with the second well, and the embedded layer is fixed at apotential that prevents carriers of the second conductivity type in theembedded layer from flowing into the second well.
 2. A semiconductormemory device according to claim 1, further comprising another impurityregion of the first conductivity type, wherein the another impurityregion is formed in the third well, the another impurity region is adrain of the load transistor, the another impurity region is an elementthat forms the cell node, the embedded layer is not formed in an areaunder the third well where the another impurity region is located, andthe semiconductor substrate is the first conductivity type.
 3. Asemiconductor memory device according to claim 1, further comprising awell contact region of the second conductivity type, wherein the wellcontact region is formed in the third well, the well contact region is acontact region that fixes a well potential of the third well, and theembedded layer contacts the third well.
 4. A semiconductor memory deviceaccording to claim 1, further comprising another embedded layer of thefirst conductivity type, wherein the another embedded layer is formedunder the second well, and the another embedded layer contacts thesecond well.
 5. A semiconductor memory device according to claim 1,wherein the first well, the second well and the third well areretrograded wells.
 6. A semiconductor memory device according to claim5, wherein each of the first well, the second well and the third wellhas a first concentration layer, a second concentration layer and athird concentration layer in the order from a top layer, wherein thefirst well further has a fourth concentration layer under the thirdconcentration layer.
 7. A semiconductor memory device according to claim1, the device element isolation region has a length ranging from 0.2 μmto 1.6 μm.
 8. A semiconductor memory device according to claim 1,wherein the second well and the third well have a depth ranging from 0.5μm to 1.2 μm.
 9. A semiconductor memory device according to claim 1,wherein the semiconductor substrate is of p-type.